Conventionally, differential pressure sensors that use sensor diaphragms for outputting signals in accordance with differences between pressures borne on one face and borne on the other face have been used as differential pressure sensors for industrial use. These differential pressure sensors are structured so as to guide liquid pressures, which will act on high-pressure-side and low-pressure-side pressure bearing diaphragms, to one side face and the other side face of a sensor diaphragm, through a pressure transmitting medium (a filled liquid) such as silicone oil, so as to detect the deformation of the sensor diaphragm as, for example, a change in a resistance value of a strain resistance gauge, to convert this change in the resistance value into an electric signal, so as to be outputted to the outside.
This type of differential pressure sensor is used when measuring, for example, a liquid surface height through detecting a pressure difference between two locations, upper and lower, within a sealed tank for storing a fluid that is to be measured, such as a high-temperature reaction tower in an oil refining plant.
FIG. 24 illustrates the critical structures in a conventional differential pressure sensor. See, for example, Japanese Unexamined Patent Application Publication No. 2005-69736. In this figure: 1-1 is a sensor diaphragm; 1-2 and 1-3 are first and second stoppers that are bonded with the sensor diaphragm 1-1 interposed therebetween; and 1-4 and 1-5 are first and second glass pedestals to which the stoppers 1-2 and 1-3 are bonded, where a sensor chip 1 is structured with the stopper 1-2 and the glass pedestal 1-4 as a first retaining member, and the stopper 1-3 and the glass pedestal 1-5 as a second retaining member. The sensor diaphragm 1-1 is made from silicon, and a strain resistance gauge is formed on a surface of the diaphragm, which is formed in a thin plate shape. The stoppers 1-2 and 1-3 are also structured from silicon.
In this sensor chip 1, a recessed portion 1-2a is formed in the stopper 1-2, where the peripheral edge portion 1-2b of the recessed portion 1-2a faces one face 1-1a of the sensor diaphragm 1-1, and the stopper 1-2 is bonded to the one face 1-1a the sensor diaphragm 1-1. A recessed portion 1-3a is formed in the stopper 1-3, where the peripheral edge portion 1-3b of the recessed portion 1-3a faces the other face 1-1b of the sensor diaphragm 1-1, and the stopper 1-3 is bonded to the other face 1-1b the sensor diaphragm 1-1.
The recessed portions 1-2a and 1-3a of the stoppers 1-2 and 1-3 have surfaces (aspherical surfaces) that are curved along the dislocation of the sensor diaphragm 1-1, where pressure introducing holes (pressure guiding holes) 1-2c and 1-3c are formed at the apex portions thereof. Pressure introducing holes (pressure guiding holes) 1-4a and 1-5a are formed in the glass pedestals 1-4 and 1-5 as well, at positions corresponding to those of the pressure guiding holes 1-2c and 1-3c of the stopper members 1-2 and 1-3.
This sensor chip 1 is contained in a sensor chamber 3 that is formed as an interior space in a sensor housing 2 (a package body made from metal). In this example, the top face of the sensor chip 1 (the top face of a glass pedestal 1-4) is in an open state, that is, the bottom face of the sensor chip 1 (the bottom face of a glass pedestal 1-5) is coated with an epoxy adhesive agent and bonded to an inner wall surface 3b of the bottom face side of the sensor chamber 3 without the top face of the sensor chip 1 being bonded to the inner wall surface 3a of the top face side of the sensor chamber 3. In the sensor housing 2, a pressure introducing path (a pressure guiding path) 2b is formed in a position corresponding to a pressure guiding hole 1-5a of the glass pedestal 1-5.
In this differential pressure sensor 100, the fluid pressure Pa passes through the pressure guiding hole 1-4a of the glass pedestal 1-4 and the pressure guiding hole 1-2c of the stopper 1-2, through a pressure transmitting medium such as silicon oil, to be applied to the one face 1-1a of the sensor diaphragm 1-1. Moreover, the fluid pressure Pb passes through the pressure introducing path 2b of the sensor housing 2 and the pressure guiding hole 1-5a of the glass pedestal 1-5 and the pressure guiding hole 1-3c of the stopper 1-3, through a pressure transmitting medium such as silicon oil, to be applied to the other face 1-1b of the sensor diaphragm 1-1.
In this case, when there is a displacement of the sensor diaphragm 1-1 when an excessively large pressure is applied to the other face 1-1a of the sensor diaphragm 1-1, the entirety of the dislocated face is supported and stopped by the curved surface of the recessed portion 1-3a of the stopper 1-3. Moreover, then when there is a displacement of the sensor diaphragm 1-1 when an excessively large pressure is applied to the other face 1-1b of the sensor diaphragm 1-1, the entirety of the dislocated face is supported and stopped by the curved surface of the recessed portion 1-2a of the stopper 1-2.
This effectively prevents accidental rupturing of the sensor diaphragm 1-1 due to the application of an excessively large pressure, through preventing excessive dislocation when an excessively large pressure is applied to the sensor diaphragm 1-1, by preventing a concentration of stresses on the peripheral edge portion of the sensor diaphragm 1-1, thus enabling an increase in the excessively large pressure guard operating pressure (withstand pressure).
However, the differential pressure sensor 100 with such a structure has a weakness in that when one face 1-1a side of the sensor diaphragm 1-1 is the high-pressure side for bearing the pressure Pa and the other face 1-1b side of the sensor diaphragm 1-1 is the low-pressure side for bearing the pressure Pb, it will break easily when a pressure (a reverse pressure) that is higher than that of the high-pressure side is applied to the low-pressure side.
That is, in this differential pressure sensor 100, when a pressure that is higher than the low-pressure side is applied to the high-pressure side, the withstand pressure is high because a state will be produced where in the junction portion 4-1 is pressed by the pressure Pa, which is a high-pressure that would act so as to separate the junction portion 4-1 between the stopper 1-2 and the sensor diaphragm 1-1 of the sensor chip 1, but when a pressure (a reverse pressure) that is higher than that of the high-pressure side is applied to the low-pressure side, a state will be produced wherein the junction portion 4-2 is pressed by only the pressure Pa, which is a pressure that is lower than the pressure Pb, which is a high-pressure that acts of so as to separate the junction portion 4-2 between the stopper 1-3 and the sensor diaphragm 1-1 of the sensor chip 1, and thus tends to cause damage.
Note that while normally one would think that in use the one face 1-1a side of the sensor diaphragm 1-1 that bears the pressure Pa will be set as the high-pressure side and the other face 1-1b side of the sensor diaphragm 1-1 that bears in the pressure Pb will be set as the low-pressure side, in a differential pressure sensor 100 with this structure there will be cases wherein the high/low relationship between the pressure Pa and the pressure Pb may become reversed, or, without the high/low relationship of the pressure Pa and the pressure Pb becoming reversed, the one face 1-1a side of the sensor diaphragm 1-1 may mistakenly be selected as the low-pressure side and the other face 1-1b side set as the high-pressure side, so merely establishing the high-pressure side and the low-pressure side cannot eliminate the weakness that the junction portions 4-1 and 4-2 in the sensor chip 1 can be separated easily.
The present invention was created in order to solve such issues, and an aspect thereof is to provide a differential pressure sensor, and a differential pressure sensor manufacturing method, able to achieve simultaneously an improvement in pressure durability performance and mitigation of thermal stress in the sensor chip.